graft: add test for reading old graftstate files with new mechanism

This tests the reading of old graftstate file using the new logic. The tests
shows that if user is in middle of a graft and then updates their mercurial to
the version where we have new graftstate format, we can still read the old graft
state format files correctly.

Differential Revision: https://phab.mercurial-scm.org/D2597

tests/test-graft.t

@@ -1373,3 +1373,40 b' also detecting that both 3 and 5 should '
   note: graft of 7:d3c3f2b38ecc created no changes to commit
 
   $ cd ..
+
+Testing the reading of old format graftstate file with newer mercurial
+
+  $ hg init oldgraft
+  $ cd oldgraft
+  $ for ch in a b c; do echo foo > $ch; hg add $ch; hg ci -Aqm "added "$ch; done;
+  $ hg log -GT "{rev}:{node|short} {desc}\n"
+  @  2:8be98ac1a569 added c
+  |
+  o  1:80e6d2c47cfe added b
+  |
+  o  0:f7ad41964313 added a
+  
+  $ hg up 0
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ echo bar > b
+  $ hg add b
+  $ hg ci -m "bar to b"
+  created new head
+  $ hg graft -r 1 -r 2
+  grafting 1:80e6d2c47cfe "added b"
+  merging b
+  warning: conflicts while merging b! (edit, then use 'hg resolve --mark')
+  abort: unresolved conflicts, can't continue
+  (use 'hg resolve' and 'hg graft --continue')
+  [255]
+
+Writing the nodes in old format to graftstate
+
+  $ hg log -r 1 -r 2 -T '{node}\n' > .hg/graftstate
+  $ echo foo > b
+  $ hg resolve -m
+  (no more unresolved files)
+  continue: hg graft --continue
+  $ hg graft --continue
+  grafting 1:80e6d2c47cfe "added b"
+  grafting 2:8be98ac1a569 "added c"